There has been a great deal of interest in the search for materials that can transfer electrical energy directly into mechanical energy, analogous to the physiology of muscles converting electrical and chemical energy into mechanical energy to produce movement. Advances in these endeavors have been achieved around the world, using materials such as poly(vinyl alcohol), ionized poly(acrylamide), poly(acrylic acid), poly(acrylic acid)-co-(poly(acrylamide), poly(2-acrylamide-2-methyl-1-propane sulfonic acid), poly(acrylic acid), poly(methacrylic acid), poly(styrene sulfonic acid), quarternized poly(4-vinyl pyridinium chloride), and poly(vinylbenzyltrimethyl ammonium chloride), to name a few. SRI International holds patents in this area (U.S. Pat. Nos. 6,707,236, 6,664,718, 6,628,040, 6,586,839, 6,583,533, 6,545,384, 6,376,971, 6,543,110), as well as the Nippon Zeon Corporation and the Nitta Corporation (U.S. Pat. No. 5,977,685). Piezoelectric materials have also been investigated for use as electrically responsive materials; however, most piezoelectric materials undergo length changes of only a fraction of one percent.
Another area of research has been electronic electroactive polymers. Ron Pelrine, et al, at SRI International, has produced electrically driven mechanochemical actuators, where the electric field is applied through flexible carbon plates, which provide for an expandable conducting surface, and an elastomeric material is sandwiched between the carbon plates (Pelrine, R. E., Kornbluh, R. D., Pei, Q., Oh, S., Joseph, J. P., “Electroactive polymers and their use in devices for conversion of electrical to mechanical energy,” U.S. Pat. No. 6,376,971 B1 [2002], WO Patent 2001006579 A2 [2001]). The elastomeric material wedged between the carbon plates acts as a flexible, movable structure when the two carbon plates, with opposing charges, are attracted and move closer to each other for the duration of the electric impulse. When the electric field is turned off, the smart material resumes its previous configuration. A good overview of electronic electroactive polymers is discussed in Scientific American, “Artificial Muscles,” [2003], 289(4), 52-59 and in the Proceedings of SPIE (4695), “Dielectric Elastomer Artificial Muscle Actuators: Toward Biomimetric Motion,” [2002], Smart Structures and Materials 2002: EAPAD, Ed.: Bar-Cohen, Y., 126-137.
The predominant research of SRI International, Mohsen Shahinpoor, and Yoseph Bar-Cohen are examples of electronic electroactive polymers; however, electronic electroactive polymers typically require high voltages, and once the configuration is reached, the smart material is static.
Another area of research has been ionic electroactive polymers. Toyoichi Tanaka, et al, observed that ionized poly(acrylamide) gels, immersed in 50% acetone and 50% water mixture, collapsed and physically shrunk in the presence of an electric field (Tanaka, T., Nishio, I., Sun, S., Tang; U., “Collapse of gels in an electric field,” Science [1982], 218(4571), 467-9). Tanaka has also investigated branched polymers (Tanaka, M., Grosberg, A. Y., Tanaka, T., “Molecular dynamics of multi-chain coulomb polymers and the effect of salt ions,” AIP Conference Proceedings [1999], 469 [Slow Dynamics in Complex Systems]) and polyampholytic hydrogels (English, A. E., Tanaka, T., Edelman, E. R., “Polymer and solution ion shielding in polyampholytic hydrogels,” Polymer [1998], 39(24), 5893-5897). Tohru Shiga, et al, found that poly(acrylic acid) gels copolymerized with poly(vinyl alcohol) and with poly(acrylamide), exhibited bending deformations in the presence of an electric field (Shiga, T., Polym. Preprints [1989], 3(1), 310-314; Shiga, T., “Polymer Gels” [1991], Plynom. Press, NY, Editor(s): DeRossi, D., 237-246). Yoshihito Osada, et al, published observations of not only types of materials that are responsive to electricity, but also types of materials that are unresponsive to an electric field (Osada, Y., Adv. Poly, Sci. [1987], 82, 3; Chen, L., Gong, J. P., Ohsedo, Y., Osada, Y., “Water-swollen hydrogels with pendant terthiophenes,” Macromolecular Chemistry and Physics [2003], 204(4), 661-665; Osada, Y., Gong, J. P., “Electrical behaviors and mechanical responses of polyelectrolyte gels,” in “Polymer Gels and Networks” [2002], 177-217. Editor(s): Osada, Y., Khokhlov, A. R., Marcel Dekker, Inc., New York, N.Y.; Osada, Y., Gong, J. P., Narita, T., “Intelligent gels,” Materials Research Society Symposium Proceedings [2000], 604 (Materials for Smart Systems III), 149-159). Osada has also developed several actuators that convert electrical energy to mechanical energy by walking and looping movements (Osada, Y., Adv. Mater. [1991], 3(2), 107; Gong, J. P., Osada, Y., “Chemical motors using gel motors,” Kagaku to Kogyo (Tokyo) [2000], 53(2), 184-187). Mohsen Shahinpoor has used electrically responsive polymers coupled with springs and other mechanical devices to improve upon electrically responsive actuators (Shahinpoor, M., “Spring-Loaded Polymeric Gel Actuators,” U.S. Pat. No. 5,389,222 [1995]). Lenore Rasmussen found that copolymers comprising cross-linked networks of methacrylic acid and 2-hydroxy methacrylate, cross-linked with cross-linking agents such as ethylene glycol dimethacrylate and 1,1,1-trimethylolpropane trimethacrylate, were superior ionic electroactive materials, with tensile strengths well above the tensile strengths of polyacrylamide type materials (Rasmussen, L., “Process for producing an electrically driven mechanochemical actuator,” U.S. Pat. No. 5,736,590, [1998]). A good overview of ionic electroactive polymers is described by Mohsen Shahinpoor in Electrochimica Acta, “Ionic Polymer-Conductor Composites as Biometric Sensors, Robotic Actuators, and Artificial Muscles—a Review,” [2003], 48(14-16), 2342-2353. As long as the electricity is on, ionic electroactive polymers typically continue to move. A relatively small amount of electricity will cause a response. However, ionic electroactive polymers must be in a wet environment in order to function.
In view of the foregoing, an alternative form of electrically driven mechanochemical actuators is desirable.